In many vehicular display devices, display areas are backlighted by light sources. An illuminator for lighting a display is proposed in JP-A-3-118593. In the illuminator, a light source is arranged so that light enters into an optical guiding plate from an entry-end surface and leaves the optical guiding plated from an exit-side surface, which is a front surface of the optical guiding plate. The optical guiding plate has a reflecting surface opposed to the exit-side surface and the reflecting surface reflects the light toward the exit-side surface. As a result, a plane area is lighted. A dial of a meter can be backlighted with this illuminator.
A rear surface of the optical guiding plate is curved such that the optical guiding plate becomes thinner toward to the end away from the light source. Therefore, a distance between the reflecting surface and the exit-side surface becomes smaller as the thickness of the optical guiding plate becomes thinner. Namely, brightness may not be uniform on the exit-side surface.
Furthermore, light emitted from the light source and traveling in the air are entered into the optical guiding plate. The light travels in the optical guiding plate and leaves it from the exit-side surface to the air. The optical guiding plate is usually made of light transmitting material, such as a transparent acrylic resign and a polycarbonate resin. A density of the air and the density of the optical guiding plate are different and therefore travel speeds of light in the air and in the optical guiding plate are different.
Namely, the travel speeds of light at entrance to the optical guiding plate and at exit from the optical guiding plate are different. Thus, the travel speed of light decreases at entrance to or at exit from the optical guiding plate. Moreover, some of the light emitted from the light source is reflected off interfaces of the optical guiding plate. Therefore, the amount of light passing through the interfaces is reduced, that is, the amount of light available for illumination is reduced.
The reduction in the amount of light causes reduction in brightness, or in uniformity of illumination on display members. To solve this problem, a brighter light source may be used. However, the cost of light source increases as the brightness increases, and the cost of the display device increases as the capacity of a light source driving circuit increases.
In another arrangement, a light source is positioned at the rear of a display member such that light emitted toward the display member and a diffusion sheet is used for diffusing light so that the display member is backlighted. To provide uniform brightness, a certain distance is required between the light source and the display member, namely, reducing the size of the display device is limited. Thus, installability of the display device in an instrument panel of the vehicle cannot be improved.